netdev - Re: [PATCH net] tcp_bbr: better deal with suboptimal GSO

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Date:   Wed, 21 Feb 2018 15:18:41 +0000
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, oleksandr@...alenko.name,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp_bbr: better deal with suboptimal GSO

On Wed, Feb 21, 2018 at 10:14 AM Neal Cardwell <ncardwell@...gle.com> wrote:

> On Wed, Feb 21, 2018 at 9:43 AM, Eric Dumazet <eric.dumazet@...il.com>
wrote:
> >
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > BBR uses tcp_tso_autosize() in an attempt to probe what would be the
> > burst sizes and to adjust cwnd in bbr_target_cwnd() with following
> > gold formula :
> >
> > /* Allow enough full-sized skbs in flight to utilize end systems. */
> > cwnd += 3 * bbr->tso_segs_goal;
> >
> > But GSO can be lacking or be constrained to very small
> > units (ip link set dev ... gso_max_segs 2)
> >
> > What we really want is to have enough packets in flight so that both
> > GSO and GRO are efficient.
> >
> > So in the case GSO is off or downgraded, we still want to have the same
> > number of packets in flight as if GSO/TSO was fully operational, so
> > that GRO can hopefully be working efficiently.
> >
> > To fix this issue, we make tcp_tso_autosize() unaware of
> > sk->sk_gso_max_segs
> >
> > Only tcp_tso_segs() has to enforce the gso_max_segs limit.
> >
> > Tested:
> >
> > ethtool -K eth0 tso off gso off
> > tc qd replace dev eth0 root pfifo_fast
> >
> > Before patch:
> > for f in {1..5}; do ./super_netperf 1 -H lpaa24 -- -K bbr; done
> >     691  (ss -temoi shows cwnd is stuck around 6 )
> >     667
> >     651
> >     631
> >     517
> >
> > After patch :
> > # for f in {1..5}; do ./super_netperf 1 -H lpaa24 -- -K bbr; done
> >    1733 (ss -temoi shows cwnd is around 386 )
> >    1778
> >    1746
> >    1781
> >    1718
> >
> > Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control")
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Reported-by: Oleksandr Natalenko <oleksandr@...alenko.name>
> > ---
> >  net/ipv4/tcp_output.c |    9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)

> Acked-by: Neal Cardwell <ncardwell@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Thank you Eric for the nice patch!